I. Field of the Invention
This invention relates to a display device using light emitting diodes (LED's), and, more particularly, to a large-sized display device including relatively small substances connected with one another.
II. Description of the Prior Art
Generally, in a display device for displaying desired patterns, such as letters, numerals, chinese characters, special symbols, illustrations, graphic patterns, etc., a plurality of light sources formed of LED's are closely disposed on a common substrate. A desired pattern is displayed by selectively supplying electric signals to the individual LED's. Since the electric signals are transmitted intermittently as short pulses, the individual LED's are driven only instantaneously. By repeatedly transmitting the electric signals at high speed, however, the pattern may be caused to look stationary with the aid of the afterimage effect of the human eye.
Recently, display devices of the aforesaid type have come to show a tendency toward enlargement in size as the amount of information to be displayed has increased with the development of measuring instruments and electronic computers. There are proposed two measures for increasing the size of display devices: mounting of a plurality of LED's on a large-sized substrate and a method in which a plurality of relatively small substrtates each having LED's mounted thereon are electrically connected with one another. The former measure has not, however, been put to practical use because of its requiring complicated driving systems as well as its deteriorating yield of production.
FIG. 1 shows a typical example of the latter measure. In this display device, as shown in FIG. 1, two substrates, for example, are connected by means of wires. More specifically, horizontally extending parallel wiring layers 12 are formed at regular intervals on a first substrate 11. Each wiring layer 12 has relatively wide regions 12a on which LED's 13 are fixed with their cathode side downward. LED's fixed on each two adjacent wiring layers are electrically connected by stitch bonding on the anode side with use of wires 14.
A second substrate 15 has wiring layers 16 which are the same as the wiring layers 12 of the first substrate 11 and LED's 17 are connected with one another by means of wires 18. The second substrate 15 is disposed in close vicinity to the first substrate 11 so as to be symmetrical with respect to an axis A--A at right angles to the wiring layers 12 and 16. Bonding pads 12b and 16b are provided respectively at the facing ends of the respective wiring layers of the substrates 11 and 15, and the wiring layers 12 and 16 are connected at these bonding pads by means of gold wires 19.
In such display device, the bonding pads 12b and 16b require a diameter of 0.2 mm or more for wire bonding. In view of the performance of currently available bonding machines, moreover, a certain measure of space (usually 0.5 mm) need be left between each bonding pad 12b or 16b and the LED 13 or 17 nearest thereto. In consideration of the bonding accuracy of the adjoining portions of the substrates, furthermore, a space of 1.5 mm or more is required between the LED's 13 and 17. With such wide space, however, the portion of a display pattern corresponding to such space will be unclear to the human eye.
Further, this second measure cannot be applied to the connection between substrates with double-layer matrix wiring structure. Conventionally, as shown in FIG. 2, one such substrate with double-layer wiring structure includes row wiring layers 22 formed on a ceramic substrate 21, an insulation layer 23 with openings formed on the row wiring layers, and a column wiring layer 24 on the insulation layer 23. LED's 25 are fixed on exposed portions of the row wiring layers, each LED being connected at its anode side with the column wiring layers 24 by means of wires 26. In the double-layer wiring substrate of such construction, as may be seen from FIG. 2, the row wiring layers 22 is embedded in the insulation layer 23 at the end portion. Accordingly, it is impossible to bond one such substrate to another by using wires in the manner shown in FIG. 1.